1. Field of the Invention
The invention pertains to the field of variable camshaft timing (VCT) systems. More particularly, the invention provides an effective means of performing the switch between operating states, with minimal disturbance to the system.
2. Description of Related Art
It is known to use closed loop control system in a variable cam timing (VCT) control system. In the variable cam timing (VCT) control system, there may be conditions when the controller must switch between open and closed loop operations. However, if this switching is not performed carefully, then a disturbance to the system may occur, resulting in poor system transient performance.
Vane type VCT systems include both xe2x80x9cCam Torque Actuated (CTA) and xe2x80x9cOil Pressure Actuatedxe2x80x9d (OPA) systems. Both systems generally operate as follows: when the control valve is at the center or xe2x80x9cnullxe2x80x9d position, oil flow in both the xe2x80x9cAdvancexe2x80x9d and the xe2x80x9cRetardxe2x80x9d chambers is blocked. The control valve at null position hydraulically locks the phaser in its current position. When the control valve is moved away from xe2x80x9cnullxe2x80x9d in one direction, oil is allowed to flow into the xe2x80x9cAdvancexe2x80x9d chamber, advancing the camshaft phase angle. When the control valve is moved away from xe2x80x9cnullxe2x80x9d in the other direction, oil is allowed to flow into the xe2x80x9cRetardxe2x80x9d chamber, retarding the camshaft phase angle.
U.S. Pat. No. 5,289,805 provides an improved VCT method involving a closed loop feedback control system. The method utilizes a hydraulic PWM spool position control and an advanced control method suitable for use in a computer program product that yields a prescribed set point tracking behavior with a high degree of robustness.
A feedback loop is used when the system parameters are within a suitable range. However, outside the range, the feedback loop may be counter productive with regard to engine control.
Referring now to FIG. 1, a prior art feedback loop 10 is shown. The control objective of feedback loop 10 is to have the VCT phaser at the correct phase (set point 12) and the phaser rate of change reduced to zero. In this state, the spool valve 14 is in its null position and no fluid flows between two fluid holding chambers of a phaser (not shown). A computer program product which utilizes the dynamic state of the VCT mechanism is used to accomplish the above state.
The VCT closed-loop control mechanism is achieved by measuring a camshaft phase shift.xcex80 16, and comparing the same to the desired set point 12. The VCT mechanism is in turn adjusted so that the phaser achieves a position which is determined by the set point 12. A control law 18 compares the set point 12 to the phase shift xcex80 16. The compared result is used as a reference to issue commands to a solenoid 20 to position the spool 14. This positioning of spool 14 occurs when the phase error (the difference between set point 12 and measured phase xcex80 (16) is non-zero.
The spool 14 is moved toward a first direction (e.g. right) if the phase error is positive (retard) and to a second direction (e.g. left) if the phase error is negative (advance). When the phase error is zero, the VCT phase equals the set point 12 so the spool 14 is held in the null position such that no fluid flows within the spool valve.
Camshaft and crankshaft measurement pulses in the VCT system are generated by camshaft and crankshaft pulse wheels 22 and 24, respectively. As the crankshaft (not shown) and camshaft (also not shown) rotate, wheels 22, 24 rotate along with them. The wheels 22, 24 possess teeth which can be sensed and measured by sensors according to measurement pulses generated by the sensors. The measurement pulses are detected by camshaft and crankshaft measurement pulse sensors 22a and 24a, respectively. The sensed pulses are used by a phase measurement device 26. A measurement of the CAM position or phase expressed as xcex80 16 is then determined. This phase measurement is then supplied to the control law 18 for giving suitable commands to reach the desired spool position.
U.S. Pat. No. 6,263,846 provides hydraulic system for adjusting cam phase. The hydraulic system uses a pair of three way hydraulic valves controllable by a controller to control the flow of liquid to the advance and retard chambers respectively. Further, the need for a spool valve is eliminated. As can be appreciated, the cam phase adjustment in this invention uses oil pressure as an actuating force.
During the operational life of the VCT system, there may be conditions when the controller must switch between open and closed loop operations or modes. Similarly, the switching occurs on the reverse in that the controller must switch from closed loop to open loop modes. The switching causes disruptions, if this switching is not performed carefully, then a disturbance to the system may occur, resulting in poor system transient performance. Therefore, it is desirous to provide a method for switching between the above two modes with minimum disturbance.
In a variable cam timing control system having conditions as to when the control system must operate in an open or closed loop mode, a method for switching between the above two modes is provided.
In the variable cam timing control system, a method for switching between the above-two modes with minimum disturbance is provided. The method is suitable for vane type variable cam timing systems including cam torque actuated (CTA) and oil pressure actuated (OPA) systems.
Accordingly, in a variable cam timing (VCT) system that has a plurality of states indicating a set of two operational modes of the VCT system, the two operational modes of the VCT system are open loop mode and closed loop mode. The system with the plurality of states includes a first state that is disposed to be transferred to a second state with the transformation based upon a set of conditions. A method is provided comprising the steps of: providing a closed feedback control loop for the VCT system to operate under the closed loop mode. During transferring from the first state to the second state, switching from a closed loop mode to an open loop mode occurs. In the open loop mode, closed feedback control loop is not used; and during transferring from the second state to the first state, switching from the open loop mode to the closed loop mode occurs; and the closed feedback control loop is used.